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Standard Template Library Programmer's Guide
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hash_map<Key, Data, HashFcn, EqualKey, Alloc>


Category: containers	Component type: type

Description

Hash_map is a Hashed Associative Container that associates objects of type Key with objects of type Data. Hash_map is a Pair Associative Container, meaning that its value type is pair<const Key, Data>. It is also a Unique Associative Container, meaning that no two elements have keys that compare equal using EqualKey.

Looking up an element in a hash_map by its key is efficient, so hash_map is useful for "dictionaries" where the order of elements is irrelevant. If it is important for the elements to be in a particular order, however, then map is more appropriate.

Example

struct eqstr
{
  bool operator()(const char* s1, const char* s2) const
  {
    return strcmp(s1, s2) == 0;
  }
};

int main()
{
  hash_map<const char*, int, hash<const char*>, eqstr> months;
  
  months["january"] = 31;
  months["february"] = 28;
  months["march"] = 31;
  months["april"] = 30;
  months["may"] = 31;
  months["june"] = 30;
  months["july"] = 31;
  months["august"] = 31;
  months["september"] = 30;
  months["october"] = 31;
  months["november"] = 30;
  months["december"] = 31;
  
  cout << "september -> " << months["september"] << endl;
  cout << "april     -> " << months["april"] << endl;
  cout << "june      -> " << months["june"] << endl;
  cout << "november  -> " << months["november"] << endl;
}

Definition

Defined in the header hash_map, and in the backward-compatibility header hash_map.h. This class is an SGI extension; it is not part of the C++ standard.

Template parameters

Parameter	Description	Default
`Key`	The hash_map's key type. This is also defined as `hash_map::key_type`.
`Data`	The hash_map's data type. This is also defined as `hash_map::data_type`.
`HashFcn`	The hash function used by the hash_map. This is also defined as `hash_map::hasher`.	`hash<Key>`
`EqualKey`	The hash_map key equality function: a binary predicate that determines whether two keys are equal. This is also defined as `hash_map::key_equal`.	`equal_to<Key>`
`Alloc`	The `hash_map`'s allocator, used for all internal memory management.	`alloc`

Model of

Unique Hashed Associative Container, Pair Associative Container

Type requirements

Key is Assignable.
EqualKey is a Binary Predicate whose argument type is Key.
EqualKey is an equivalence relation.
Alloc is an Allocator.

Public base classes

None.

Members

Member	Where defined	Description
`key_type`	Associative Container	The `hash_map`'s key type, `Key`.
`data_type`	Pair Associative Container	The type of object associated with the keys.
`value_type`	Pair Associative Container	The type of object, `pair<const key_type, data_type>`, stored in the hash_map.
`hasher`	Hashed Associative Container	The `hash_map`'s hash function.
`key_equal`	Hashed Associative Container	Function object that compares keys for equality.
`pointer`	Container	Pointer to `T`.
`reference`	Container	Reference to `T`
`const_reference`	Container	Const reference to `T`
`size_type`	Container	An unsigned integral type.
`difference_type`	Container	A signed integral type.
`iterator`	Container	Iterator used to iterate through a `hash_map`. [1]
`const_iterator`	Container	Const iterator used to iterate through a `hash_map`.
`iterator begin()`	Container	Returns an `iterator` pointing to the beginning of the `hash_map`.
`iterator end()`	Container	Returns an `iterator` pointing to the end of the `hash_map`.
`const_iterator begin() const`	Container	Returns an `const_iterator` pointing to the beginning of the `hash_map`.
`const_iterator end() const`	Container	Returns an `const_iterator` pointing to the end of the `hash_map`.
`size_type size() const`	Container	Returns the size of the `hash_map`.
`size_type max_size() const`	Container	Returns the largest possible size of the `hash_map`.
`bool empty() const`	Container	`true` if the `hash_map`'s size is `0`.
`size_type bucket_count() const`	Hashed Associative Container	Returns the number of buckets used by the `hash_map`.
`void resize(size_type n)`	Hashed Associative Container	Increases the bucket count to at least `n`.
`hasher hash_funct() const`	Hashed Associative Container	Returns the `hasher` object used by the `hash_map`.
`key_equal key_eq() const`	Hashed Associative Container	Returns the `key_equal` object used by the `hash_map`.
`hash_map()`	Container	Creates an empty `hash_map`.
`hash_map(size_type n)`	Hashed Associative Container	Creates an empty `hash_map` with at least `n` buckets.
hash_map(size_type n, const hasher& h)	Hashed Associative Container	Creates an empty `hash_map` with at least `n` buckets, using `h` as the hash function.
hash_map(size_type n, const hasher& h, const key_equal& k)	Hashed Associative Container	Creates an empty `hash_map` with at least `n` buckets, using `h` as the hash function and `k` as the key equal function.
template <class InputIterator> hash_map(InputIterator f, InputIterator l) [2]	Unique Hashed Associative Container	Creates a hash_map with a copy of a range.
template <class InputIterator> hash_map(InputIterator f, InputIterator l, size_type n) [2]	Unique Hashed Associative Container	Creates a hash_map with a copy of a range and a bucket count of at least `n`.
template <class InputIterator> hash_map(InputIterator f, InputIterator l, size_type n, const hasher& h) [2]	Unique Hashed Associative Container	Creates a hash_map with a copy of a range and a bucket count of at least `n`, using `h` as the hash function.
template <class InputIterator> hash_map(InputIterator f, InputIterator l, size_type n, const hasher& h, const key_equal& k) [2]	Unique Hashed Associative Container	Creates a hash_map with a copy of a range and a bucket count of at least `n`, using `h` as the hash function and `k` as the key equal function.
`hash_map(const hash_map&)`	Container	The copy constructor.
`hash_map& operator=(const hash_map&)`	Container	The assignment operator
`void swap(hash_map&)`	Container	Swaps the contents of two hash_maps.
pair<iterator, bool> insert(const value_type& x)	Unique Associative Container	Inserts `x` into the `hash_map`.
template <class InputIterator> void insert(InputIterator f, InputIterator l) [2]	Unique Associative Container	Inserts a range into the `hash_map`.
`void erase(iterator pos)`	Associative Container	Erases the element pointed to by `pos`.
`size_type erase(const key_type& k)`	Associative Container	Erases the element whose key is `k`.
`void erase(iterator first, iterator last)`	Associative Container	Erases all elements in a range.
`void clear()`	Associative Container	Erases all of the elements.
`const_iterator find(const key_type& k) const`	Associative Container	Finds an element whose key is `k`.
`iterator find(const key_type& k)`	Associative Container	Finds an element whose key is `k`.
`size_type count(const key_type& k) const`	Unique Associative Container	Counts the number of elements whose key is `k`.
pair<const_iterator, const_iterator> equal_range(const key_type& k) const	Associative Container	Finds a range containing all elements whose key is `k`.
pair<iterator, iterator> equal_range(const key_type& k)	Associative Container	Finds a range containing all elements whose key is `k`.
data_type& operator[](const key_type& k) [3]	`hash_map`	See below.
bool operator==(const hash_map&, const hash_map&)	Hashed Associative Container	Tests two hash_maps for equality. This is a global function, not a member function.

New members

These members are not defined in the Unique Hashed Associative Container and Pair Associative Container requirements, but are specific to hash_map.

Member	Description
data_type& operator[](const key_type& k) [3]	Returns a reference to the object that is associated with a particular key. If the `hash_map` does not already contain such an object, `operator[]` inserts the default object `data_type()`. [3]

Notes

[1] Hash_map::iterator is not a mutable iterator, because hash_map::value_type is not Assignable. That is, if i is of type hash_map::iterator and p is of type hash_map::value_type, then *i = p is not a valid expression. However, hash_map::iterator isn't a constant iterator either, because it can be used to modify the object that it points to. Using the same notation as above, (*i).second = p is a valid expression.

[2] This member function relies on member template functions, which at present (early 1998) are not supported by all compilers. If your compiler supports member templates, you can call this function with any type of input iterator. If your compiler does not yet support member templates, though, then the arguments must either be of type const value_type* or of type hash_map::const_iterator.

[3] Since operator[] might insert a new element into the hash_map, it can't possibly be a const member function. Note that the definition of operator[] is extremely simple: m[k] is equivalent to (*((m.insert(value_type(k, data_type()))).first)).second. Strictly speaking, this member function is unnecessary: it exists only for convenience.